1. Field of the Invention
This invention relates to Yagi antennas, and more particularly, to Yagi antennas having a pair of opposed linear arrays of parallel parasitic elements.
2. Related Art
Yagi antennas are used for various high-frequency applications such as the reception of television signals, point-to-point communications, and certain types of military communications. They are becoming increasingly used for what is commonly referred to as wireless or cableless television transmission by which numerous signals are transmitted over a design frequency band.
A basic Yagi antenna has a single driven element, usually a half-wave dipole, which is driven from a source of, or which drives a sink of electromagnetic energy. Arrayed with the dipole are certain non-driven or parasitic elements. These typically include a reflector element on one side of the dipole and one or more director elements on the other side of the dipole.
All of these elements are typically positioned along an antenna axis with the director elements extending in what is referred to herein as the transmission direction from the dipole. The transmission direction is that direction to which electromagnetic energy is to be transmitted, or from which signal energy is to be received.
It is known to use parasitic elements in other configurations. For instance, placement of a sleeve around the dipole or elements on each side of and parallel to the dipole provides an antenna having a satisfactory gain or directivity over a relatively broad frequency range, as is stated in U.S. Pat. No. 5,061,944. This arrangement of parasitic elements appears to allow the array of directors on the antenna axis to be about 25% shorter than would otherwise be required.
It is also known to provide parasitic arrays parallel to and adjacent to the distal end of the main array on the antenna axis to improve the directivity of the antenna, as is disclosed in U.S. Pat. No. 3,218,645. The described antenna is said to provide an increase in gain of 60%, which is equivalent to a decrease in length of about 38% compared to a standard Yagi antenna for the same gain.
While this known art is effective in increasing the gain or decreasing the length for a given gain of a Yagi antenna, it is further desirable to have even shorter antennas for the same gain. It is yet further desirable to have an antenna that is relatively inexpensive and simple to manufacture.